Organización Internacional del Bambú y el Ratán

Organización Internacional del Bambú y el Ratán

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Superelasticity by reversible variants reorientation in a Ni-Mn-Ga microwire with bamboo grains







Wang Z. L.; Zheng P.; Nie Z. H.; Ren Y.; Wang Y. D.; Muellner P.; Dunand D. C.





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Ni2MnGa; Size effect; Superelasticity; Shape-memory effect; Twinning; Oligocrystalline materials

The link between microstructure and mechanical properties is investigated for a superelastic Ni-Mn-Ga microwire with 226 mu m diameter, created by solidification via the Taylor method. The wire, which consists of bamboo grains with tetragonal martensite matrix and coarse gamma precipitates, exhibits fully reversible superelastic behavior up to 4% tensile strain. Upon multiple tensile load unload cycles, reproducible stress fluctuations of similar to 3 MPa are measured on the loading superelastic stress plateau of similar to 50 MPa. During cycles at various temperatures spanning similar to 70 to 55 degrees C, the plateau stress decreases from 58 to 48 MPa near linearly with increasing temperature. Based on in situ synchrotron X-ray diffraction measurements, we conclude that this superelastic behavior is due to reversible martensite variants reorientation (i.e., reversible twinning) with lattice rotation of similar to 13 degrees. The reproducible stress plateau fluctuations are assigned to reversible twinning at well-defined locations along the wire. The strain recovery during unloading is attributed to reverse twinning, driven by the internal stress generated on loading between the elastic gamma precipitates and the twinning martensite matrix. The temperature dependence of the twining stress on loading is related to the change in tetragonality of the martensite, as measured by X-ray diffraction. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.